Biology Sample Question Paper Sheet - Class Xii, Central Board Of Secondary Education, 2010 Page 38
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39iii.
Two successive mitotic division - an eight nucleate embryo sac
½ × 3
iv.
Cell wall formation takes place after nuclear divisins
½ × 7
v.
Three cells group together at the micropylar end - egg apparatus with an egg
cell and two synergids
vi.
Three cells group together at the chalazal end - antipodal cells
vii.
The remaining two nuclei move to the centre - fuse to form secondary nucleus.
5
OR
Zygote ———
A
½
Cleavage ——
B 2 celled stage
½
C 4 celled stage
½
D Morula with vitelline membrane
1
E Blastocyst with trophoblast and inner cell mass
1
F Implantation f blastocys
½
G In the endometrium within wal
½
30.
i.
Regulator gene, Promoter gene. Operator gene and structural gene
1.
An inducible operon where Lactose is the inducer and it is the substrate for the enzyme
B-galactosidase
2.
Three structural genes (z, y, a) which transcribe the polycistronic mRNA
3.
z codes for - galactosidase, y for permease and a for transacetylase
4.
Near the structural genes is the promoter gene where RNA polymerase binds for
transcription
5.
An operator gene as a switch near the promoter where a repressor always binds.
6.
Repressor protein coded by the i gene prevents the RNA polymerase from transcribing
by binding to the operator
7.
Lactose, an inducer inactivates the repressor and prevents it from binding to the operator.
8.
Allows an access for the RNA polymerase to the promoter
9.
Transcription takes place
38
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